Aqueous coating compositions for environmental etch resistant coatings

ABSTRACT

Environmental etch resistant coatings are described. The coatings are derived from aqueous, curable coating compositions comprising: 
     (a) a carbamate-functional grafted acrylic polymer that comprises the reaction product of a first acrylic polymer and a second acrylic polymer, with at least one of the first and second acrylic polymers having groups that impart water dispersibility to the grafted acrylic polymer, and at least one of the first and second acrylic polymers having carbamate functionality; and 
     (b) a second component comprising a compound having a plurality of functional groups that are reactive with the carbamate functionality.

FIELD OF THE INVENTION

This invention relates to aqueous, curable coating compositions andcoatings produced therefrom. In particular, the invention relates totopcoat coatings, especially those with high gloss and/or high DOI(distinctness-of-image) such as those used as automotive topcoats.

BACKGROUND OF THE INVENTION

The effect of the coating process on the environment and the effect ofthe environment on coatings have increasingly shaped the coatings art inthe last few decades. The industry has put considerable effort intodeveloping coatings with materials that will be less harmful toward theenvironment, such as coatings containing lower levels of volatileorganic compounds for a lower impact on the environment during theirapplication. As another concern, it has been difficult to devise such acoating that will also have the desired resistance to environmentaldegradation.

One mode of environmental degradation that has attracted attention inrecent years is environmental etch. "Environmental etch" is the namegiven to a kind of surface pitting and spotting that is thought to bedue to the attack of environmental acids on the coating. Environmentaletch manifests itself as spots or marks on or in the coating that oftencannot be rubbed out. Environmental etch is particularly a problem wherethe coating is a high gloss and/or a high DOI (distinctness-of-image)coating, since defects in the surface of such a glossy or highlyreflective coating are very noticeable. High gloss and/or high DOIcoatings are widely utilized in the coatings art. The automotiveindustry has made extensive use of such coatings, often ascolor-plus-clear composite coatings for automotive body panels.Automotive coatings are especially likely to exhibit environmental etchbecause the significant outdoor exposure and the frequent heat buildupin the coated article are factors favoring etch degradation.

Prior art coating compositions such as high-solids enamels, althoughknown for their durability and weatherability when used in exteriorpaints, have not provided the desired level of resistance toenvironmental etch when used as topcoat coatings. Environmental etch hasbeen a particular concern with prior art high gloss or high DOIcoatings. Compositions such as polyurethanes, acid-epoxy systems and thelike have been proposed for use as topcoat coatings. However, many priorart systems suffer from disadvantages such as coatability problems,compatibility problems with underlying coatings, or high content ofvolatile organic compounds. Moreover, very few one-pack coatingcompositions have been found that provide satisfactory resistance toenvironmental etch, especially under the demanding conditions to whichautomotive coatings are subjected. Thus, there exists a continuing needfor curable coating compositions with low content of volatile organiccompounds that provide satisfactory resistance to environmental etch.

SUMMARY OF THE INVENTION

It has now been discovered that an environmental etch resistant coatingcan be derived from an aqueous, curable coating composition comprising:

(a) a carbamate-functional grafted acrylic polymer that comprises thereaction product of:

(i) a first acrylic polymer, having a functionality (I), and

(ii) a second acrylic polymer, having a functionality (II) that isreactive toward the functionality (I) of the first acrylic polymer,

where one or both of the first and second acrylic polymers has groupsthat impart water dispersibility to the grafted acrylic polymer and oneor both of the first and second acrylic polymers has carbamatefunctionality; and

(b) a compound having a plurality of functional groups that are reactivewith the carbamate functionality.

The coating, when cured, provides a tough, durable, and attractivefinish that is highly resistant to environmental etch. The coatingcomposition can be effectively applied as a one-pack system without thenecessity of mixing reactive materials just prior to application as in atwo-pack system. The coatings of the invention provide improvedresistance to acid etch over previous waterborne systems. The coating isparticularly useful as a coating having a 20° gloss, as defined by ASTMD523-89, of at least 80 or a DOI, as defined by ASTM E430-91, of atleast 80, or both, and is especially useful as a clear coat in acomposite color-plus-clear coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The aqueous, curable coating compositions of the invention contain botha carbamate-functional grafted acrylic polymer and a compound having aplurality of functional groups that are reactive with the carbamatefunctionality of the acrylic polymer. The grafted acrylic polymer ismade by reacting together a first acrylic polymer having a functionality(I) and a second acrylic polymer having a functionality (II) that isreactive toward the functionality (I) of the first acrylic polymer. Oneor both of the first and second acrylic polymers has carbamatefunctionality, and one or both of the first and second acrylic polymershas groups that impart water dispersibility to the grafted acrylicpolymer.

The carbamate functionality may be on only one of the first and secondacrylic polymers. In a preferred embodiment the carbamate functionalityis on both the first acrylic polymer and the second acrylic polymer. Thecarbamate functionality can be introduced onto the acrylic polymer orpolymers in a variety of ways. One way to prepare such polymers is touse in the polymerization reaction an acrylic monomer having a carbamatefunctionality in the ester portion of the monomer. Such monomers arewell-known in the art and are described, for example in U.S. Pat. Nos.3,479,328, 3,674,838, 4,126,747, 4,279,833, and 4,340,497, thedisclosures of which are incorporated herein by reference. For example,one method of synthesis involves reaction of a hydroxy ester with ureato form the carbamyloxy carboxylate (i.e., carbamate-modified acrylate).Another method of synthesis reacts an α,β-unsaturated acid ester with ahydroxy carbamate ester to form the carbamyloxy carboxylate. Yet anothertechnique involves formation of a hydroxyalkyl carbamate by reacting aprimary or secondary amine or diamine with a cyclic carbonate such asethylene carbonate. The hydroxyl group on the hydroxyalkyl carbamate isthen esterified by reaction with acrylic or methacrylic acid to form thecarbamate-functional monomer. Other methods of preparingcarbamate-modified acrylic monomers are described in the art and can beutilized as well. The acrylic monomer can then be polymerized along withother ethylenically-unsaturated monomers using techniques well-known inthe art.

An alternative route for introducing the carbamate functionality ontothe acrylic polymer or polymers is to react an already-formed acrylicpolymer with another component to adduct a carbamate-functional group tothe acrylic polymer backbone, as described in U.S. Pat. No. 4,758,632,the disclosure of which is incorporated herein by reference. Onetechnique for preparing carbamate-functional polymers involves thermallydecomposing urea to ammonia and HNCO in the presence of ahydroxy-functional acrylic polymer to form as the reaction product acarbamate-functional acrylic polymer. Another technique involvesreacting the hydroxyl group of a hydroxyalkyl carbamate with theisocyanate group of an isocyanate-functional acrylic polymer to form thecarbamate-functional acrylic polymer. Isocyanate-functional acrylics areknown in the art and are described, for example in U.S. Pat. No.4,301,257, the disclosure of which is incorporated herein by reference.Ethylenically unsaturated isocyanate monomers are well-known in the artand include meta-isopropenyl-α,α-dimethylbenzyl isocyanate (sold byAmerican Cyanamid as TMI®) and isocyanatoethyl methacrylate. Yet anothertechnique is to react cyclic carbonate groups on an acrylic with ammoniato form the carbamate-functional acrylic polymer. Cycliccarbonate-functional acrylic polymers are known in the art and aredescribed, for example, in U.S. Pat. No. 2,979,514, the disclosure ofwhich is incorporated herein by reference. Another approach is atranscarbamylation reaction of a hydroxy-functional polymer with analkyl carbamate. A more difficult, but feasible, way of preparing thepolymer would be to trans-esterify an acrylate polymer with ahydroxyalkyl carbamate.

In one preferred embodiment, the carbamate functionality comprises thestructure --L--O--C(═O)--NHR. R represents H, alkyl, preferably of 1 to6 carbon atoms, or cycloalkyl, preferably up to 6 ring carbon atoms. Itis to be understood that the terms alkyl and cycloalkyl are to includesubstituted alkyl and cycloalkyl, such as halogen-substituted alkyl orcycloalkyl. Substituents that will have an adverse impact on theproperties of the cured material, however, are to be avoided.

L represents a divalent linking group, preferably an aliphatic linkinggroup of 1 to 8 carbon atoms, a cycloaliphatic linking group, or anaromatic linking group of 6 to 10 carbon atoms. Examples of L include##STR1## --(CH₂)--, --(CH₂)₂ --, --(CH₂)₄ --, and the like. In onepreferred embodiment, --L-- is represented by --COO--L'--, where L' is adivalent linking group.

L' may be a divalent aliphatic linking group, preferably of 1 to 8carbon atoms, e.g., --(CH₂)--, --(CH₂)₂ --, --(CH₂)₄ --, and the like,or a divalent cycloaliphatic linking group, preferably up to 8 carbonatoms, e.g., cyclohexyl, and the like. However, other divalent linkinggroups can be used, depending on the technique used to prepare thepolymer. For example, if a hydroxyalkyl carbamate is adducted onto anisocyanate-functional acrylic polymer, the linking group L' wouldinclude an --NHCOO-- urethane linkage as a residue of the isocyanategroup.

Additionally, one or both of the first and second acrylic polymers hasthereon hydrophilic groups that impart water dispersibility to thegrafted acrylic polymer. While in some cases it may be advantageous tohave the groups that impart water dispersibility to the grafted acrylicpolymer on both the first and the second acrylic polymers, in othercases it will be preferred to have such groups on only the secondacrylic polymer. The dispersions formed from having all of thehydrophilic groups on one polymer may have improved applicationcharacteristics. The groups that impart water dispersibility to thegrafted acrylic polymer can be anionic, cationic, or nonionic. Anionicand nonionic groups are preferred for many applications because of thetendency of the cationic (e.g., amine) groups on the polymer to causeyellowing in the cured coating. Carboxylic acid groups are particularlypreferred as the groups that impart water dispersibility to the graftedacrylic polymer.

Similarly to the introduction of the carbamate groups onto the acrylicpolymer or polymers, the groups that impart water dispersibility can beintroduced either by polymerization with an ethylenically unsaturatedmonomer containing such a group or by further reaction of the formedacrylic polymer to adduct the hydrophilic group onto the backbone.Reactive monomers that can be employed during polymerization of thepolymer to introduce the groups that impart water dispersibility includeethylenically unsaturated acids and acid anhydrides, ethylenicallyunsaturated amines, and ethylenically unsaturated polyethers. Usefulethylenically unsaturated acids include α,β-olefinically unsaturatedmonocarboxylic acids containing 3 to 5 carbon atoms, α,β-olefinicallyunsaturated dicarboxylic acids containing 4 to 6 carbon atoms and theiranhydrides and monoesters, and unsaturated sulfonic acids.Representative examples include acrylic, methacrylic, and crotonic acid;fumaric, maleic, and itaconic acids and anhydrides and their monoesterssuch as maleic aid monomethyl ester and maleic acid monohexyl ester; andacrylamido methylpropanesulfonic acid. Useful ethylenically unsaturatedamines include t-butylaminoethyl methacrylate and dimethylaminoethylmethacrylate. Useful ethylenically unsaturated polyethers include thealkoxy poly(oxyalkylene) alcohol esters or amides of α,β-olefinicallyunsaturated monocarboxylic acids containing 3 to 5 carbon atoms.

Another technique involves reacting the groups that impart waterdispersibility onto the acrylic polymer after the polymerizationreaction. This may be accomplished, for example, by synthesizing anhydroxyl-functional acrylic and adducting the hydroxyl groups with anacid anhydride. Another method of preparation would be by synthesizingan isocyanate-functional acrylic and adducting the isocyanate groupswith alkoxy poly(oxyalkylene) alcohol or alkoxypoly(oxyalkylene) amine.

The alkoxy poly(oxyalkylene) alcohol or alkoxy poly(oxyalkylene) amineemployed in either forming the monomer described above or in reactingonto the acrylic polymer after the polymerization reaction can beobtained by the alkoxylation of monohydric alcohols with ethylene oxideor mixtures of ethylene oxide with other epoxides of up to ten carbonatoms, such as propylene oxide or butylene oxide. The residue of thealkoxy poly(oxyalkylene) alcohol or amine contained in the acrylicpolymer can be represented by D(CH(R₁)CH₂ O--)_(n) R₂, and is eitheralkoxy polyoxyethylene or an alkoxy polyoxyethylene/polyoxyalkylenecopolymer, having a degree of polymerization of n, n being an integerfrom one to one thousand. D is O in the case of the alkoxypoly(oxyalkylene) alcohol and NH in the case of the amine. Preferably, nis an integer from 20 to 200; more preferably, from 40 to 70. R₁ is thuseither hydrogen or a mixture of hydrogen and alkyls of one to eightcarbon atoms. It is particularly advantageous for R₁ to be eitherhydrogen or a mixture of hydrogen and alkyls of one to three carbonatoms. R₂ is an alkyl of one to thirty carbon atoms. R₂ is preferably analkyl of one to ten carbon atoms. In one embodiment, R₁ can be hydrogenand R₂ can be methyl.

In addition to ethylenically unsaturated monomers having the carbamatefunctionality or the groups used to impart water dispersibility to thegrafted acrylic polymer, or those bearing reactive groups that will besites for the introduction of these groups onto the acrylic polymer asdescribed above, one or more other ethylenically unsaturated monomersare employed in forming the first and second acrylics of the invention.Such monomers for copolymerization with acrylic monomers are known inthe art. They include esters, nitriles, or amides of α,β-olefinicallyunsaturated monocarboxylic acids containing 3 to 5 carbon atoms;diesters of α,β-olefinically unsaturated dicarboxylic acids containing 4to 6 carbon atoms; vinyl esters, vinyl ethers, vinyl ketones, vinylamides, and vinyl compounds of aromatics and heterocycles.Representative examples include acrylic and methacrylic acid amides andaminoalkyl amides; acrylonitrile and methacrylonitriles; esters ofacrylic and methacrylic acid, particularly those with saturatedaliphatic and cycloaliphatic alcohols containing 1 to 20 carbon atomssuch as methyl, ethyl, propyl, butyl, 2-ethylhexyl, isobutyl, isopropyl,cyclohexyl, tetrahydrofurfuryl, and isobornyl acrylates andmethacrylates; acrylates or methacrylates having hydroxy, isocyanato, orother functional groups, such as hydroxyalkyl acrylates andmethacrylates, glycidyl esters of methacrylic or acrylic acid such asglycidyl methacrylate, and aminoalkyl esters of methacrylic or acrylicacid like N,N-dimethylaminoethyl (meth)acrylate; fumaric, maleic, anditacontc diesters, like maleic acid dimethyl ester and maleic aciddihexyl ester; vinyl acetate, vinyl propionate, vinyl ethyl ether, andvinyl ethyl ketone; styrene, a-methyl styrene, vinyl toluene, and2-vinyl pyrrolidone.

The copolymers may be prepared by using conventional techniques, such asfree radical polymerization, cationic polymerization, or anionicpolymerization, in, for example, a batch or semi-batch process. Forinstance, the polymerization may be carried out by heating theethylenically unsaturated monomers in bulk or in organic solution oraqueous dispersion in the presence of a free radical source, such as anorganic peroxide or azo compound and, optionally, a chain transfer agentfor a batch process; or, alternatively, the monomers and initiator(s)may be fed into the heated reactor at a controlled rate in a semi-batchprocess.

Typical free radical sources are organic peroxides such as dialkylperoxides, peroxyesters, peroxydicarbonates, diacyl peroxides,hydroperoxides, and peroxyketals; and azo compounds such as2,2'-azobis(2-methylbutanenitrile) and1,1'-azobis(cycohexanecarbonitrile). Typical chain transfer agents aremercaptans such as octyl mercaptan, n- or tert-dodecyl mercaptan,thiosalicyclic acid, mercaptoacetic acid, and mercaptoethanol;halogenated compounds, and dimeric alpha-methyl styrene.

The free radical polymerization is usually carried out at temperaturesfrom about 20° C. to about 200° C., preferably from 90° C. to 170° C.The reaction may conveniently be done at the temperature at which thesolvent or solvent mixture refluxes, although reflux is not necessary tothe reaction. The initiator should be chosen to match the temperature atwhich the reaction is carried out, so that the half-life of theinitiator at the reaction temperature should preferably be no more thanthirty minutes.

If solvents are used in the polymerization reaction, the solvents usedare preferably water or water-soluble or -miscible organic solvents thatcan function as cosolvents. A cosolvent is useful to aid in dispersionof the components and in flow during cure of the composition. Examplesof useful solvents include methyl ethyl ketone, methyl isobutyl ketone,xylene, n-amyl acetate; and cosolvents such as N-methylpyrrolidone andglycol ethers like ethylene glycol butyl ether, ethylene glycol butylether acetate, diethylene glycol butyl ether, ethylene glycol2-ethylhexyl ether, propylene glycol methyl ether, propylene glycolmethyl ether acetate, propylene glycol butyl ether, and dipropyleneglycol butyl ether.

The solvent or solvent mixture is generally heated to the reactiontemperature and the monomers and initiator(s) used to make the firstacrylic polymer or the second acrylic polymer are added at a controlledrate over a period of time, usually between 2 and 6 hours. A chaintransfer agent or additional solvent may be added concurrently with themonomers and initiator(s). The mixture is usually held at the reactiontemperature after the additions for a period of time to complete thereaction. Optionally, additional initiator may be added during thelatter stages of the addition or after the addition is completed,usually over a period of one to sixty minutes, to ensure completeconversion.

The first and second acrylic polymers will generally have a molecularweight of 2000-20,000, and preferably from 4000-6000. Molecular weightas used herein means number average molecular weight, and can bedetermined by gel permeation chromatography (GPC) using a polystyrenestandard. The weight per equivalent of carbamate functionality, willgenerally be between 200 and 1500, and preferably between 300 and 700.

The grafted acrylic polymer of the invention is made by reactingtogether the first acrylic polymer having the functionality (I) and thesecond acrylic polymer having the functionality (II) that is reactivetoward the functionality (I) of the first acrylic polymer. There aremany pairs of functional groups that could be employed for this purpose.For example, if functionality (I) is an isocyanate group, thenfunctionality (II) can be a group with an active hydrogen such as anhydroxyl or primary or secondary amino group. Alternatively,functionality (I) could be an acid or anhydride group whilefunctionality (II) could be an epoxy or hydroxyl group. In anothermethod of reacting the first and second acrylic polymers together,functionality (I) could be epoxy while functionality (II) could be acarboxyl group, a primary or secondary amino group, or phenolic group.It is also possible for functionality (I) to be carbamate functionalityand functionality (II) to be, for example, methylol, methoxy,isobutyoxy, or siloxane groups. In one preferred embodiment,functionality (I) is an epoxy group and functionality (II) is acarboxylic acid group, and it is particularly preferred in such a casethat the groups that impart water dispersibility to the grafted acrylicpolymer are carboxylic acid groups on the second acrylic polymer.

The invention contemplates various methods of reacting together thefirst and second acrylic polymers. In one scheme, the first and secondacrylic polymers are formed in separate reaction vessels, then mixedtogether and reacted under appropriate conditions. In a second andpreferred method, the first acrylic polymer is formed and then thesecond acrylic polymer is polymerized in the presence of the firstacrylic polymer. After polymerization is completed, the polymer mixtureis subjected to conditions conducive to the reaction between thefunctionalities (I) and (II) in order to form the grafted acrylicpolymer. In another contemplated method, the second acrylic polymer isagain polymerized in the presence of the previously formed first acrylicpolymer, but in this case the reaction between functionalities (I) and(II) take place during the formation of the second acrylic polymer. Theresult of this reaction scheme is the formation of the second acrylicpolymer and the grafted acrylic polymer in the same step. In each case,the amount of functionality (I) and functionality (II) on theirrespective polymers are chosen to avoid gelation of the reactionproduct. Gelation can be avoided by limiting the extent of reactionbetween functionalities (I) and (II), but it is usually preferred tolimit the amount of one or the other of functionalities (I) and (II) ontheir respective polymers.

The practitioner will readily recognize that many combinations of groupsfor functionalities (I) and (II) will serve the purpose of reactingtogether the first and second acrylics to form the grafted acrylicpolymer. While reaction conditions might vary, such conditions would bereadily apparent to the skilled artisan from experience orstraightforward testing. It may be useful to employ catalysts such asare known in the art, as for example to use tin catalyst in reactingisocyanate groups with alcohol groups or to use triphenyl phosphine oramines to catalyze reaction of epoxy groups with carboxylic acid groups.

The coating compositions of the invention contain aqueous dispersions ofthe grafted acrylic polymer. In one method of practicing the invention,the first and second acrylic polymers are prepared by emulsionpolymerization and the reaction forming the grafted acrylic polymer iscarried out in the aqueous medium. Alternatively, the first and secondacrylic polymers may be formed in organic medium and then reactedtogether to form the grafted acrylic polymer. A sufficient amount ofdeionized water is then added to the grafted acrylic polymer to form anaqueous dispersion. In another method, the first and second acrylicpolymers are polymerized in organic medium. The formed acrylic polymersmay then be emulsified by addition of a sufficient quantity of deionizedwater to form a dispersion. The reaction forming the grafted acrylicpolymer may then be carried out in the aqueous dispersion. In formingthe aqueous dispersions of the acrylic polymers, anionic or cationicgroups can be formed by salting with the appropriate counter ion.Anionic groups on the acrylic can be salted with ammonia or amines suchas dibutylamine, dimethylethanolamine, or N-ethyl morpholine. Cationicgroups on the acrylic can be salted with carboxylic acids such as formicacid, acetic acid, or lactic acid.

The composition of the invention is cured by a reaction of thecarbamate-functional grafted acrylic polymer component (a) with acomponent (b) having a plurality of functional groups that are reactivewith the carbamate groups on component (a). Such functional groupsinclude active methylol or alkoxy groups on aminoplast resins or onother compounds, such as phenol/formaldehyde adducts; blocked isocyanategroups; or siloxane groups. Examples of (b) compounds include curingagents or crosslinking agents such as melamine formaldehyde resins(including monomeric or polymeric melamine resin and partially or fullyalkylated melamine resin), urea resins (e.g., methylol ureas such asurea formaldehyde resin, alkoxy ureas such as butylated ureaformaldehyde resin), polyanhydrides (e.g., polysuccinic anhydride),N-methylol acrylamide emulsions, isobutoxy methyl acrylamide emulsions,and polysiloxanes (e.g., trimethoxy siloxane). Aminoplast resins such asmelamine formaldehyde resins or urea formaldehyde resins are especiallypreferred.

The curing agent or crosslinking agent (b) is used in an amountsufficient to produce well-cured, solvent-resistant films. For example,when the preferred melamine curing agents are used, they are used at atotal of 3 to 30 weight percent, based upon the total weight of the (a)and (b) components.

The coating composition used in the practice of the invention may alsoinclude a catalyst to enhance the cure reaction. For example, whenaminoplast compounds, especially monomeric melamines, are used ascomponent (b), a strong acid catalyst may be utilized to reduce thetemperature or time needed for full cure. Such catalysts are well-knownin the art and include, for example, p-toluenesulfonic acid,dinonylnaphthalene disulfonicacid, dodecylbenzenesulfonic acid, phenylacid phosphate, monobutyl maleate, butyl phosphate, and hydroxyphosphate ester. These catalysts may be blocked, for example, with anamine, or they may be unblocked. Other catalysts that may be useful inthe composition of the invention include Lewis acids, zinc salts, andtin salts such as dibutyl tin dilaurate or dibutyl tin oxide.

Additional water and or cosolvent may be added during preparation of thecoating compositions. In a preferred embodiment of the invention, thewater and organic solvent, including any cosolvent, is present in thecoating composition in an amount of from about 0.01 weight percent toabout 99 weight percent, preferably from about 40 weight percent toabout 90 weight percent, and more preferably from about 50 weightpercent to about 70 weight percent.

Other materials well-known to the coatings artisan, for example,surfactants, fillers, stabilizers, wetting agents, dispersing agents,adhesion promoters, UV absorbers, light stabilizers such as HALS,antioxidants, and rheology control agents, may be incorporated into thecoating composition. The amount of these materials used must becontrolled to avoid adversely affecting the coating characteristics.

The coating compositions according to the present invention preferablyform the outermost layer or layer of coating on a coated substrate.Preferably, the instant coating compositions are applied over one ormore layers of primer coatings. For example, the coating compositions ofthe invention may be used as an automotive topcoat coating applied overa layer of electrocoat primer and/or primer surfacer.

When the present coating compositions are used as topcoat coatings, theypreferably have a 20° gloss, as defined by ASTM D523-89, of at least 80or a DOI, as defined by ASTM E430-91, of at least 80, or both. Suchgloss and DOI are particularly useful in providing an automotive finishthat will appeal to the buyer of the vehicle. Topcoat coatings may beone coat pigmented coatings or may be a color-plus-clear compositecoating. The coating composition of the present invention, if used as aone coat pigmented coating or the color coating of a color-plus-clearcomposite coating, will include one or more pigments well-known in theart, such as inorganic pigments like titanium dioxide, carbon black, andiron oxide pigments, or organic pigments like azo reds, quinacridones,perylenes, copper phthalocyanines, carbazole violet, monoarylide anddiarylide yellows, naphthol orange, and the like. In a preferredembodiment, the coating composition of the present invention is theclearcoat of a color-plus-clear composite coating. The clearcoat may beapplied over a color coat according to the invention or may,alternatively, be applied over a color coat of a formulation alreadyknown in the art.

Pigmented color coat or basecoat compositions for such compositecoatings are well-known in the art and do not require explanation indetail herein. Polymers known in the art to be useful in basecoatcompositions include acrylics, vinyls, polyurethanes, polycarbonates,polyesters, alkyds, and polysiloxanes. Preferred polymers includeacrylics and polyurethanes. In one preferred embodiment of theinvention, the basecoat composition also utilizes a carbamate-functionalacrylic polymer. Basecoat polymers are preferably crosslinkable, andthus comprise one or more type of crosslinkable functional groups. Suchgroups include, for example, hydroxy, isocyanate, amine, epoxy,acrylate, vinyl, silane, and acetoacetate groups. These groups may bemasked or blocked in such a way so that they are unblocked and availablefor the cross-linking reaction under the desired curing conditions,generally elevated temperatures. Useful crosslinkable functional groupsinclude hydroxy, epoxy, acid, anhydride, silane, and acetoacetategroups. Preferred crosslinkable functional groups include hydroxyfunctional groups and amino functional groups.

Basecoat polymers may be self-crosslinking or may require a separatecrosslinking agent that is reactive with the functional groups of thepolymer. When the polymer comprises hydroxy functional groups, forexample, the crosslinking agent may be an aminoplast resin, apolyisocyanate and blocked polyisocyanate resin (including anisocyanurate, biuret, or the reaction product of a diisocyanate and apolyol having less than twenty carbon atoms), and an acid or anhydridefunctional crosslinking agent.

Coating compositions can be coated on the article by any of a number oftechniques well-known in the art. These include, for example, spraycoating, dip coating, roll coating, curtain coating, and the like. Forautomotive body panels, spray coating is preferred. When the coatingswill be relatively thick, they are usually applied in two or more coatsseparated by a time sufficient to allow some of the water and/or solventevaporate from the applied coating layer ("flash"). The coats as appliedare usually from 1 to 3 mils of the coating composition, and asufficient number of coats are applied to yield the desired finalcoating thickness.

Where a color-plus-clear composite coating is applied to the preparedsubstrate, the color coat is usually applied in one or two coats, thenallowed to flash, and the clear coat is then applied to the uncuredcolor coat in one or two coats. The two coating layers are then curedsimultaneously. Preferably, the cured base coat layer is 0.5 to 1.5 milsthick and the cured clear coat layer is 1 to 3 mils, more preferably 1.6to 2.2 mils thick.

The coating compositions described herein are preferably subjected toconditions so as to cure the coating layers. Although various methods ofcuring may be used, thermal-curing is preferred. Generally, thermalcuring is effected by exposing the coated article to elevatedtemperatures provided primarily by radiative heat sources. Curingtemperatures will vary depending on the particular blocking groups usedin the crosslinking agents, however they generally range between 93° C.and 177° C. In a preferred embodiment, the cure temperature is between135° C. and 165° C. In another preferred embodiment, a blocked acidcatalyst is included in the composition and the cure temperature isbetween 115° C. and 140° C. In a different preferred embodiment, anunblocked acid catalyst is included in the composition and the curetemperature is between 80° C. and 100° C. The curing time will varydepending on the particular components used and physical parameters,such as the thickness of the layers. Typical curing times range from 15to 60 minutes, and preferably 15-25 minutes at the target temperature.

The invention is further described in the following examples.

Preparation I. Carbamate-functional Grafted Acrylic

A three-necked 5-1 round bottom flask was fitted with an agitator at thecenter neck and a thermal couple at one of the side necks to monitor thereaction temperature. A nitrogen purge line was also fed through thisneck. The second side neck was fitted with a Claissen adapter and watercooled condenser.

A mixture of 213.3 g propylene glycol methyl ether and 240.0 gdiethylene glycol butyl ether were charged to the flask and heated to110° C. A monomer mixture I of 25.9 g glycidyl methacrylate, 177.6 gn-butyl acrylate, 413.4 g n-butyl methacrylate, 413.4 g cyclohexylmethacrylate, 122.6 g styrene, and 188.5 g t-butyl peroxy2-ethylhexanoate was mixed in a separate addition pot and then added tothe flask over a three-hour period at constant temperature. A separatemonomer mixture II of 738.2 g carbamate propyl methacrylate (96%), 738.2g propylene glycol methyl ether, and 0.19 g 4-methoxy phenol was addedsimultaneously over the three-hour period. After the additions werecomplete, the addition pots were rinsed with a total of 31.6 g propyleneglycol methyl ether, which was then added to the flask. Following therinse, a mixture of 18.6 g t-butyl peroxy 2-ethylhexanoate and 13.2 gpropylene glycol methyl ether was added to the flask over a period of 30minutes. The contents of the flask were then held at a constanttemperature of 108° C. for 30 minutes.

A monomer mixture III of 9.9 g n-butyl acrylate, 22.4 g n-butylmethacrylate, 22.4 g cyclohexyl methacrylate, 6.7 g styrene, 47.7 gacrylic acid, and 5.1 g t-butyl peroxy 2-ethylhexanoate was mixed in theaddition pot and then added to the flask over a one-hour period atconstant temperature. A separate monomer mixture IV of 40.4 g carbamatepropyl methacrylate (96%), 40.4 g propylene glycol methyl ether, and0.01 g 4-methoxy phenol was added simultaneously over the one-hourperiod. After the additions were complete, the addition pots were rinsedwith a total of 31.6 g propylene glycol methyl ether, which was thenadded to the flask. Next, 2.5 g t-butyl peroxy 2-ethylhexanoate and 30.6g propylene glycol methyl ether were added to the flask over a period of30 minutes. The content of the flask were maintained at constanttemperature for an additional 30 minutes. Finally, 0.4 g triphenylphosphine and 11.1 g propylene glycol methyl ether were added to flaskand the temperature was maintained until acid number indicated that allof the epoxy functionality had theoretically been consumed.

The contents of the flask were cooled to 88° C. and 41.9 gdimethylethanolamine, 24.5 g Tinuvin® 123, and 43.2 g Tinuvin® 384B werestirred in. The resulting mixture was transferred to a plastic pail.Deionized water was added with stirring until a dispersion having anonvolatile content of 43.3% was obtained.

Preparation II. Hydroxyl-Functional Grafted Acrylic

A three-necked 5-1 round bottom flask was fitted with an agitator at thecenter neck and a thermal couple at one of the side necks to monitor thereaction temperature. A nitrogen purge line was also fed through thisneck. The second side neck was fitted with a Claissen adapter and watercooled condenser.

A mixture of 213.3 g propylene glycol methyl ether and 240.0 gdiethylene glycol butyl ether were charged to the flask and heated to110° C. A monomer mixture I of 25.9 g glycidyl methacrylate, 177.6 gn-butyl acrylate, 413.4 g n-butyl methacrylate, 413.4 g cyclohexylmethacrylate, 122.6 g styrene, 439.2 g hydroxyethyl acrylate, 188.5 gt-butyl peroxy 2-ethylhexanoate, and 153.9 g propylene glycol methylether was mixed in a separate addition pot and then added to the flaskover a three-hour period at constant temperature. After the addition wascomplete, the addition pot was rinsed with 20.1 g propylene glycolmethyl ether, which was then added to the flask. Following the rinse, amixture of 18.6 g t-butyl peroxy 2-ethylhexanoate and 13.2 g propyleneglycol methyl ether was added to the flask over a period of 30 minutes.The contents of the flask were then held at a constant temperature of108° C. for 30 minutes.

A monomer mixture II of 9.9 g n-butyl acrylate, 22.4 g n-butylmethacrylate, 22.4 g cyclohexyl methacrylate, 6.7 g styrene, 47.7 gacrylic acid, 24.2 g hydroxyethyl acrylate, 5.1 g t-butyl peroxy2-ethylhexanoate, and 96.5 g propylene glycol methyl ether was mixed inthe addition pot and then added to the flask over a one-hour period atconstant temperature. The addition pot was rinsed with 51.1 g propyleneglycol methyl ether, which was then added to the flask. Next, 2.5 gt-butyl peroxy 2-ethylhexanoate and 30.6 g propylene glycol methyl etherwere added to the flask over a period of 30 minutes. The content of theflask were maintained at constant temperature for an additional 30minutes. Finally, 0.4 g triphenyl phosphine and 11.1 g propylene glycolmethyl ether were added to flask and the temperature was maintaineduntil the acid number indicated that all of the epoxy functionality hadtheoretically been consumed.

The contents of the flask were cooled to 88° C. and 41.9 gdimethylethanolamine, 24.5 g Tinuvin® 123, and 43.2 g Tinuvin® 384B werestirred in. The resulting mixture was transferred to a plastic pail.Deionized water was added with stirring until a dispersion having anonvolatile content of 39.4% was obtained.

EXAMPLE 1 Aqueous Clear Coating Composition

A clear coating composition was prepared by combining the followingmaterials in order:

355.4 g carbamate functional grafted acrylic (Preparation I)

21.4 g Cymel® 327 (high imino methylated melamine from AmericanCyanamid)

14.4 g Silwet® 7602 (silicone leveling agent, 10% in ethylene glycolbutyl ether from Union Carbide, Danbury, Conn.)

9.0 g Tegoflow® 425 (flow and leveling agent, 25% in ethylene glycolbutyl ether from Tego Chemie Service USA, Hopewell, Va.)

EXAMPLE 2 Aqueous Clear Coating Composition

A clear coating composition was prepared by combining the followingmaterials in order:

375 g carbamate functional grafted acrylic (Preparation I)

18.2 g hexamethoxymethylated melamine

4.0 g Silwet® 7602 (silicone leveling agent, 10% in ethylene glycolbutyl ether from Union Carbide, Danbury, Conn.)

3.0 g Tegoflow® 425 (flow and leveling agent, 25% in ethylene glycolbutyl ether from Tego Chemie Service USA, Hopewell, Va.)

5.4 g Nacure® XP267 (blocked acid catalyst solution from KingIndustries)

72.5 g deionized water

COMPARATIVE EXAMPLE A Aqueous, Clear Coating Composition

A clear coating composition was prepared by combining the followingmaterials in order:

282.9 g hydroxy functional grafted acrylic (Preparation II)

24.4 g Cymel® 327 (high imino methylated melamine from AmericanCyanamid)

21.4 g Silwet® 7602 (silicone leveling agent, 10% in ethylene glycolbutyl ether from Union Carbide, Danbury, Conn.)

14.4 g Tegoflow® 425 (flow and leveling agent, 25% in ethylene glycolbutyl ether from Tego Chemie Service USA, Hopewell, Va.)

40.0 g butyl Carbitol®

COMPARATIVE EXAMPLE B Aqueous, Clear Coating Composition

A clear coating composition was prepared by combining the followingmaterials in order:

1555.4 g carbamate functional grafted acrylic (Preparation 1)

83.2 g hexamethoxy methylated melamine

16.0 g Silwet® 7602 (silicone leveling agent, 10% in ethylene glycolbutyl ether from Union Carbide, Danbury, Conn.)

12.0 g Tegoflow® 425 (flow and leveling agent, 25% in ethylene glycolbutyl ether from Tego Chemie Service USA, Hopewell, Va.)

20.6 g Nacure® XP267 (blocked acid catalyst solution from KingIndustries)

45.0 g butyl Carbitol®

276 g deionized water

The coating compositions of Examples 1 and 2 and Comparative Examples Aand B were sprayed in two coats (one minute flash between coats) oversteel panels that had been previously sprayed with a high solids blackacrylic pigmented basecoat and flashed for ten minutes. The clear coatedpanel was flashed for ten minutes in a 120° F. oven. The oventemperature was increased over 45 minutes to the bake temperatureindicated in the table below and the panels were cured at thattemperature for 20 minutes.

Film builds:

basecoat 0.8 to 1.0 mil (20.3 to 25.4 microns)

clearcoat 1.4 to 1.8 mil (35.6 to 45.7 microns)

The coated panels were subjected to 16 weeks of severe weatheringconditions in Jacksonville, Fla., with the results indicated in thetable. The numbers indicate etch ratings on a scale where 1=no visibleetch, 10=severe etch.

    ______________________________________                                        Example          138° C. Cure                                                                       149° C. Cure                              ______________________________________                                        Example 1        6           5                                                Example 2        8           5                                                Comparative Example A                                                                          10+         10+                                              Comparative Example B                                                                          10          10                                               ______________________________________                                    

The invention has been described in detail with reference to preferredembodiments thereof. It should be understood, however, that variationsand modifications can be made within the spirit and scope of theinvention.

We claim:
 1. An aqueous, curable coating composition comprising:(a) acarbamate-functional grafted acrylic polymer that comprises the reactionproduct of:i) a first acrylic polymer having thereon a functionality(I); (ii) a second acrylic polymer having thereon a functionality (II)that is reactive toward the functionality (I) of the first acrylicpolymer, wherein:at least one of the first and second acrylic polymershas thereon groups that impart water dispersibility to the graftedacrylic polymer, and at least one of the first and second acrylicpolymers has thereon carbamate functionality; and (b) a compound havinga plurality of functional groups that are reactive with the carbamatefunctionality.
 2. A coating composition according to claim 1 wherein thegroups that impart water dispersibility to the grafted acrylic polymerare artionic groups.
 3. A coating composition according to claim 2wherein the groups that impart water dispersibility to the graftedacrylic polymer are carboxylic acid groups.
 4. A coating compositionaccording to claim 1 wherein the groups that impart water dispersibilityto the grafted acrylic polymer are nonionic groups.
 5. A coatingcomposition according to claim 1 wherein the groups that impart waterdispersibility to the grafted acrylic polymer are on the second acrylicpolymer.
 6. A coating composition according to claim 5 wherein thefunctionality (I) is epoxy functionality and both the functionality (II)and the groups that impart water dispersibility are carboxylic acidgroups.
 7. A coating composition according to claim 5 wherein thecarbamate functionality is on both the first and the second acrylicpolymers.
 8. A coating composition according to claim 1 wherein thegroups that impart water dispersibility to the grafted acrylic polymerare on both the first and the second acrylic polymers.
 9. A coatingcomposition according to claim 1 wherein the carbamate functionality ison only one of the first and second acrylic polymers.
 10. A coatingcomposition according to claim 1 wherein the carbamate functionality ison both the first and the second acrylic polymers.
 11. A coatingcomposition according to claim 1 wherein the carbamate functionalitycomprises the structure

    --L--O--C(═O)--NHR,

wherein L represents a divalent linking group, and R represents H,alkyl, or cycloalkyl.
 12. A coating composition according to claim 11wherein the R is hydrogen.
 13. A coating composition according to claim11 wherein the R is an alkyl of 1 to 6 carbon atoms.
 14. A coatingcomposition according to claim 11 wherein the R is cycloalkyl with 6ring carbon atoms.
 15. A coating composition according to claim 11wherein L has the formula

    --COO--L'--,

wherein L' is a divalent linking group.
 16. A coating compositionaccording to claim 1 wherein component (b) has functional groupsselected from the group consisting of siloxane, blocked isooyanate,alkoxy, and methylol functional groups.
 17. A coating compositionaccording to claim 16 wherein component (b) has functional groupsselected from the group consisting of alkoxy and methylol functionalgroups.
 18. A coating composition according to claim 1 wherein component(b) is an aminoplast resin.
 19. A coating composition according to claim18 wherein the aminoplast resin is a melamine formaldehyde resin.
 20. Acoating composition according to claim 19 wherein the melamineformaldehyde resin is fully or partially alkylated.
 21. A coatingcomposition according to claim 1 wherein the carbamate-functionalgrafted acrylic polymer is produced according to a method comprisingpolymerizing the second acrylic polymer in the presence of the firstacrylic polymer.